1. Statement of the Technical Field
The inventive arrangements relate generally to transformers and more particularly to toroidal transformers.
2. Description of the Related Art
Transformers are passive electrical devices that transform alternating or intermittent electric energy in one circuit into energy of a similar type in another circuit, commonly with altered values of voltage and current. The ratio of input voltage to output voltage in a particular transformer device is generally determined by the number of turns in the primary coil as compared to the secondary coil. There is a small loss associated with this transformation that is made up of two components. The first source of loss is referred to as “core” loss (also called no-load loss). This type of loss results from the magnetizing and de-magnetizing of the core during normal operation of the transformer. The second loss component is called coil or load loss, because the efficiency losses occur in the primary and secondary coils of the transformer. Coil loss is the result of resistance that exists in the winding materials.
For a particular transformer having a particular turns ratio it is well known that an air core will result in lower efficiency because it has a permeability of 1.0 (the terms permeability and permittivity as used herein should be understood to mean relative permeability and relative permittivity, respectively). Other types of dielectric core materials will behave similarly if they also have a relative permeability close to 1.0. Conversely, ferromagnetic materials, which have higher permeability values, are often used as core materials to increase the inductance achieved for a particular coil configuration.
Transformers can be wound around cores having a variety of shapes ranging from simple cylindrical rods to donut-shaped toroids. Toroids are advantageous in this regard since they substantially contain the magnetic field produced by the transformer within the core region so as to limit RF leakage and avoid coupling and interference with other nearby components.
In miniature RF circuitry, transformers tend to be implemented as surface mount devices or coupled planar spirals formed directly on the surface of an RF substrate. However, planar spiral transformers suffer from a serious drawback in that they do not substantially contain the magnetic field that they produce. In contrast, torroidal transformers can effectively contain the magnetic field of the transformer. However, implementation of toroids in miniaturized RF circuitry has presented practical difficulties that have typically required them to be implemented as surface mount components.
Regardless of the exact design of a surface mount component, the circuit board real estate required for such components has become a significant factor contributing to the overall size of RF systems. In fact, passive surface mount devices can typically comprise 80% of a substrate surface area. This causes the surface area of the substrate to be large, while the thickness remains relatively small. This is not an effective use of board real estate.
U.S. Pat. No. 5,781,091 to Krone, et al discloses an electronic inductive device and method for manufacturing same in a rigid copper clad epoxy laminate. The process involves drilling a series of spaced holes in an epoxy laminate, etching the copper cladding entirely off the board, positioning epoxy laminate over a second laminate, positioning a toroidal ferromagnetic core within each of the spaced holes, and filling the remainder of each hole with a fiber-filled epoxy. This technique involves numerous additional processing steps that are not normally part of the conventional steps involved in forming a conventional epoxy PWB. These additional steps naturally involve further expense. Further, such techniques are poorly suited for use with other types of substrates, such as ceramic types described below.
Glass ceramic substrates calcined at 850˜1,000 C are commonly referred to as low-temperature co-fired ceramics (LTCC). This class of materials have a number of advantages that make them especially useful as substrates for RF systems. For example, low temperature 951 co-fire Green Tape™ from Dupont® is Au and Ag compatible, and it has a thermal coefficient of expansion (TCE) and relative strength that are suitable for many applications. Other LTCC ceramic tape products are available from Electro-Science Laboratories, Inc. of 416 East Church Road, King of Prussia, Pa. 19406-2625, USA. Manufacturers of LTCC products typically also offer metal pastes compatible with their LTCC products for defining metal traces and vias.
The process flow for traditional LTCC processing includes (1) cutting the green (unfired) ceramic tape from roll, (2) removing the backing from the green tape, (3) punching holes for electrical vias, (3) filling via holes with conductor paste and screening print patterned conductors, (4) stacking, aligning and laminating individual tape layers, (5) firing the stack to sinter powders and densify, and (6) sawing the fired ceramic into individual substrates.
LTCC processing requires that materials that are co-fired are compatible chemically and with regard to thermal coefficient of expansion (CTE). Typically, the range of commercially available LTCC materials has been fairly limited. For example; LTCC materials have been commercially available in only a limited range of permittivity values and have not generally included materials with permeability values greater than one. Recently, however, developments in metamaterials have begun to expand the possible range of materials that can be used with LTCC. Further, new high-permeability ceramic tape materials that are compatible with standard LTCC processes have now become commercially available.